1. Field of the Invention
The present invention relates to a method of producing seamless pipes by carrying out a plug rolling procedure or a piercing procedure. In this method, a hollow ingot is expanded and stretched to form a bloom in a box-groove rolling mill by carrying out at least two roll passes, and using a stationary piercer or plug supported on a rod. The bloom is drawn off from the rod in a direction opposite the rolling direction after each roll pass by means of a driving unit and is rotated by 90.degree. about its longitudinal axis before carrying out a subsequent pass.
2. Description of the Related Art
In known rolling methods of producing seamless steel pipes, a distinction is made between longitudinal rolling procedures and cross rolling procedures. In contrast to the cross rolling procedure, the longitudinal rolling procedure requires an essentially round and closed or box-groove which must be adapted to the desired cross section of the pipe. The present invention deals with a longitudinal rolling procedure using an internal tool, i.e., a procedure which has become known as the plug rolling procedure or piercing procedure.
Plug rolling installations have been substantially developed by Stiefel. Stiefel-type rolling mills originally included a piercing cross rolling installation, a stretching cross rolling installation, a Stiefel-type plug rolling installation, and at least one reeling installation which were arranged in front of a reducing rolling mill train as the finishing stage. Since a considerable loss in temperature occurred in the pipe during the many stages from the piercing rolling mill to the reducing rolling mill train, the pipes were generally reheated to reducing rolling temperature after reeling rolling in a furnace provided especially for this purpose.
The original Stiefel-type plug rolling installation had rolls with a plurality of adjacent grooves cut into the rolls. Later, single-groove plug rolling installations were developed because the multiple-groove rolls resulted in inaccuracies of the rolled product because of bending or sagging of the rolls.
In known plug rolling mills, a hollow ingot is rolled in preferably two to three passes into a bloom with the use of a stationary plug which is supported on a rod. After each pass, the bloom is pulled by means of a driving unit from the rod in a direction against the rolling direction, and the bloom is turned by 90.degree. before being introduced into the next pass. The plugs are exchanged after each pass.
It is currently common to use plugs having a short working length, wherein the diameter of the plugs is greater than the internal diameter of the hollow ingot. An operation in which the hollow ingot has a greater internal diameter than the diameter of the first plug is also known. In the first case, the hollow ingots are expanded by means of the plug which results in more favorable gripping conditions of the rolls. The plugs having a short working length are economical and greater wall thickness reductions can be achieved because of the favorable gripping conditions. The plug diameter of the second pass is usually slightly greater than that of the first pass, while the third plug may have a diameter which is equal to or greater than the diameter of the second plug.
At present, modern pipe rolling procedures generally include the following steps: piercing rolling, main stretching and finish stretching, wherein the pipe is finish-rolled with the application of heat. Compared to this pipe rolling procedure, the known plug rolling procedure has considerable disadvantages. These disadvantages are due to the fact that even after a third plug pass, the blooms still have increased wall thickness portions extending in longitudinal direction of the bloom, so that it is necessary to smoothen the blooms after plug rolling in a reeling installation. In most cases, two reeling installations are used in order to equalize the increased wall thickness portions; this is necessary because the rolling speed in such installations is very low. The reeling installations usually constitute the bottleneck of the rolling mill. Since the temperature of the blooms drops substantially as a result of the low rolling speeds and the lengthy rolling procedure, the blooms are usually reheated prior to rolling in sizing rolling mill trains, reducing rolling mill trains or stretching and reducing rolling mill trains. As a result, the plug rolling mill is very expensive and uneconomical as compared to other rolling mills.